Space.com (June 27, 2011)
"An asteroid the size of a tour bus will fly past Earth today (June 27) so closely it will be beneath some of the planet's satellites.
"The rock, named asteroid 2011 MD will zoom by just 7,500 miles (12,000 km) above the planet, making a sharp turn forced by Earth's gravity before winging off into space again. The flyby will occur at about 1 p.m. EDT (1700 GMT).
"There is no risk of an impact, NASA scientists said. The space rock, estimated to be between 29 to 98 feet (9 to 30 meters) wide, is likely too small to survive a plunge through our atmosphere anyway. An asteroid this size, if it were mostly stony, would break apart and burn up before hitting the surface. Iron-heavy space rocks are better at surviving the fiery entry, however...."
There was quite a lot of science going on while 2011 MD whipped by Earth: and the odds of anything getting hit in the flyby were really, really low. It helps that the asteroid's orbit isn't in the ecliptic - the plane that planets orbit in. More or less.
The probability that it would hit something was not, however, zero. The asteroid shot by Earth about 7,500 miles out - closer than the uneven ring of geosynchronous satellites. They're 22,236 miles away, over the equator. Which isn't in the ecliptic, either - and that's another topic.
The good news is that 2011 MD wasn't going to hit Earth - and even if it had, it wouldn't have made it all the way to the surface. Probably.
The bad news is that, sooner or later, some hunk or rock, metal, or ice will hit Earth's atmosphere. And get all the way down to the surface. At which point kinetic energy in the asteroid/comet/whatever will be converted into heat. Quite a great deal of heat.
About a century back something exploded over Siberia. It wasn't a particularly big explosion, and there weren't any towns or cities nearby.
Eventually, something's going to hit Earth, near a city - or the rock will be big enough so that it won't matter where it comes down. Something big falling out of the sky probably helped kill off the dinosaurs.
City-Busting Asteroids, Budget Woes, and CongressThe American Congress has budget worries just now, so someone's almost certainly going to get the idea that they'll save money by cutting funding for science projects: like looking for asteroids. I've posted about that sort of thing before. (May 16, 2011)
Right now, that probably won't make any difference. Each year, the odds of some bit of debris from space falling on our heads is quite low. Same goes for each decade.
When it comes to centuries and millennia, though - that's different.
The Case of the Missing WitnessesSo, why don't we have historical records of a massive explosion that wiped out a civilization? These days, information about that sort of thing would be covered by Reuters and other global news agencies, data gathered and stored in archives all over the world - and probably retained for quite a while.
But what if the only witnesses didn't survive? The Siberian explosion didn't leave a crater - we might never know why some collection of ruins was abandoned.
Anyway, written records only go back a few thousand years - and that's another topic.
- "Mineral X From Outer Space!"
(May 9, 2011)
- "Asteroid Apophis: Probably Won't Hit Earth in 2036 or 2068"
(March 10, 2011)
- "Dinosaurs, Three Major Asteroid Impacts, and Massive Volcanic Eruptions"
(August 29, 2010)
- "Asteroid 1999 RQ36 Won't Hit Earth in 2182: Probably"
(July 30, 2010)
- "Proposed Commission for Planetary Defense: H.R. 5587 Makes Sense"
(July 20, 2010)